Polymeric materials have many applications in packaging structures. They are used as films, sheets, lidstock, pouches, tubes and bags. These polymeric materials may be employed as a single layer or one or more layers in a structure. Unfortunately, there are countless polymeric materials available. Furthermore, resin suppliers frequently have a tendency to claim many more applications for a product than the product is actually suitable for. In addition, in view of the specialized applications and processing problems that are encountered despite the suppliers' claims, one skilled in the art can not tell whether a particular resin will be suitable for an application unless tested. However, for various reasons there are frequently drawbacks to the use of many of these polymeric materials. For example, ethylene vinyl alcohol is an excellent oxygen barrier material for use in packaging food products. However, this polymeric material can be affected by moisture that is present in the atmosphere or the packaged product. As a result, it is frequently found that some polymeric materials are better for certain applications than others.
One area where there is a need for suitable resins in film applications is in the area of heat shrinkable films. Heat shrinkable polymeric films are commonly used in packaging meats, particularly primal meat cuts and other large pieces of meat. While this description will mainly detail the usage of films for packaging meat and meat by-products, it will be understood that these films are also suitable for packaging a myriad of other products, including food products, and non-food products, for example, dentifrices, cosmetics and pharmaceuticals.
Some of the films embodying the present invention are intended to be used by meat packers in the form of heat shrinkable bags with one opened end, which bags are closed and sealed after insertion of the meat. After the product is inserted, air is usually evacuated from the package and the open end of the bag is closed. Suitable methods of closing the bag include heat sealing, metal clips, adhesives, etc. Heat is applied to the bag once sealing is completed to initiate shrinkage of the bag about the meat.
In subsequent processing of the meat, the bag may be opened and the meat removed for further cutting of the meat into user cuts, for example, for retail cuts or for institutional use.
Suitable shrink bags must satisfy a number of criteria. Many bag users seek a bag that is capable of surviving the physical process of filling, evacuating, sealing and heat shrinking. For example, during the shrinking process great stress can be placed on the film by the sharp edges of bone in the meat. The bag must also have sufficient strength to survive the material handling involved in moving the large cuts of meat, which may weigh a hundred pounds or more, along the distribution system. Because many food products including meat deteriorate in the presence of oxygen and/or water, it is desirable that the bags have a barrier to prevent the infusion of deleterious gases and/or the loss or addition of moisture.
Conventional packaging for many products has frequently been made of multiple layer films having at least three layers. These multiple layer films are usually provided with at least one core layer of either an oxygen barrier material such as a vinylidene chloride copolymer, ethylene vinyl alcohol, a nylon or a metal foil, preferably aluminum. Heat shrinkable meat bags, for example, have generally used vinylidene chloride copolymers. The copolymer of the vinylidene chloride may, for example, be a copolymer with vinyl chloride or methyl acrylate. Collapsible dispensing containers in the form of tubes may or may not use one or more foil layers. The foil layers in addition to supplying an oxygen barrier also provide the dispensing tube with "deadfold", i.e., the property of a collapsible dispensing tube when squeezed to remain in the squeezed position without bouncing back. Collapsible dispensing tubes employing a foil layer are disclosed in U.S. Pat. No. 3,172,571 and U.S. Pat. No. 3,347,419, the disclosures of which are incorporated herein by reference. However, collapsible dispensing tubes do not require a foil layer. They may employ only one or more layers of thermoplastic or polymeric materials. Examples of such tubes are disclosed in U.S. Pat. No. 4,418,841 and U.S. Pat. No. 4,986,053, the disclosures of which are incorporated herein by reference. Methods of making collapsible dispensing tubes are well known and are disclosed in the above and other U.S. patents. Generally, foilless plastic tubes have a body wall which can be a single layer plastic sheet or film which can be extruded in tubular form and cut into desired lengths. Multilayer plastic sheet and film can be made by lamination, including coextrusion coating, processes, or by coextrusion processes such as cast coextrusion through a flat die, or tubular coextrusion through a tubular die. Single and multilayer sheet or film that has been cast or laminated typically is shaped about an elongated cylindrical mandrel and sealed to itself along a side seam to form a tubular body. Single and multilayer sheet or film that is respectively extruded or coextruded through a tubular die can be extruded in near final dimensional and vacuum sized to final dimension. Single or multilayer sheet and film can be coextruded and blown as a larger tubular form, which can be cut lengthwise and formed into a tube as would be a flat sheet or film. The formed tubular body wall is joined to a head typically having a neck with a dispensing orifice, and a shoulder. One end of the tubular body is joined to the shoulder of the head. The head can be pre-formed by compression or injection molding. Usually, the tubular body is inserted into a die and a head is injection molded onto the end of the tubular body such that the head and tubular body are fused or bonded together. The die which forms the head can be one which is shaped to form a head having an integral cap having an integrally formed hinge. Typically, the tube head is sealed by a cap, filled through the tube's lower open end, and sealed at that end. While collapsible dispensing tubes are generally made as described above, some collapsible dispensing tubes and some containers are made by an extrusion blow molding process in which a single or multilayer parison or preform is extruded and then blown in a mold into the desired shape of the finished container, for example, one having an integral body wall, shoulder, neck and closed bottom. Some containers including collapsible dispensing tubes, are made by injection molding such that they have an integral body wall, shoulder, and neck which can have an integrally formed hinge and cap, and an open bottom.
Outer layers of films and body walls of containers used in packaging food products can be any suitable polymeric material such as linear low density polyethylene, low density polyethylene, blends of these polyethylenes, and ionomers, including sodium and zinc ionomers. Such ionomers include Surlyn, ethylene vinyl acetate etc. In conventional shrink bags, the outer layers are generally linear low density polyethylene or blends thereof. Suitable outer layers for meat bags are taught by U.S. Pat. No. 4,457,960 to Newsome, the disclosures of which are incorporated herein by reference.
While conventional films have been suitable for many applications, it has been found that there is a need for films that, for example, are stronger and more easily processed than conventional films. In meat bags, there is a need for films and bags that have superior toughness and sealability and the ability to undergo cross-linking without undue deterioration. Thus, it is an object of the present invention to provide improved structures, including single and multilayer films, sheets, lidstock, and containers, for example, pouches, tubes and bags. In particular, there is a need for structures for use in shrink bags wherein the shrink bags are capable of withstanding production stresses and the shrink process.
It has also been found that there is a need for containers, for example, collapsible dispensing containers or tubes having good bond strength between a propylene polymer layer or component and an ethylene polymer component. Thus, another object of this invention is to provide improved polymeric structures with improved bonding properties. For example, it is known that in the packaging industry, it has been difficult to bond a structure, such as a collapsible dispensing tube head or layer, or simply a layer made of a propylene polymer, such as polypropylene or a propylene ethylene copolymer, directly to a structure, for example, a layer made of an ethylene polymer, for example, a low density polyethylene ("LDPE"). While it is known to add an ethylene polymer to a propylene polymer structure to improve adhesion to the ethylene polymer structure, it has heretofore been necessary to add a major amount of the ethylene polymer to sufficiently improve the adhesion. This has been undesirable. In adding a major amount, the desired properties of the propylene polymer have been significantly diminished. For example, collapsible dispensing tube heads having an integral flip top cap integrally joined to the head by a living hinge are usually made of a propylene polymer, e.g., polypropylene or a propylene ethylene copolymer, because the material properties of the polypropylene permit the hinge to be flexed repeatedly over time without cracking or breaking. Adding a major amount of a polyethylene to the propylene polymer employed to form the tube head to obtain satisfactory adhesion to a polyethylene body wall layer results in diminished hinge flexibility and/or early hinge failure. Further, since tube heads made of propylene polymer heretofore could not be satisfactorily bonded directly to a collapsible dispensing tube body wall layer of polyethylene, the packaging industry has been limited in types of heads and head materials that can be employed, and in the types of body wall layer materials that can be bonded to propylene heads. It therefore is a main objective of this invention to provide good bonding of a structure, for example, a collapsible dispensing tube head, or a layer, comprised of a propylene polymer, to a structure, for example, a tube body wall layer comprised of ethylene polymer.
Another objective of the invention is to provide a packaging structure or film having a layer of propylene polymer which can be, or which is satisfactorily bonded to a contiguous layer of an ethylene polymer.
Another object of the invention is to provide a container, for example, a collapsible dispensing container or a collapsible dispensing tube head comprised of a propylene polymer, joined or which can be joined with good bond strength directly to a structure, for example, a layer, or a body wall or side wall layer of a container or tube, where the layer is comprised of a polyethylene.
Another object of this invention is to provide an above-mentioned collapsible dispensing container or tube which does not stress crack between its first ethylene polymer body wall layer and its propylene polymer head after the dispensing container or tube has been exposed to stress crack inducing agents.
Another object of the invention is to provide a collapsible dispensing container or tube such as mentioned above, whose head has an integral cap joined directly to the head by an integral living hinge which can undergo 10,000 flexing cycles without undergoing stress fracture or failure.
Yet another object of this invention is to provide a method for improving the adhesion between the above-mentioned structures without the use of an adhesive.